The spectral sensitizing technique of adding certain sensitizing dyes to a silver halide photographic emulsion to expand its natural light-sensitive wavelength region towards the longer wavelengths is well known in the art for preparing silver halide color photographic emulsions.
The degree of spectral sensitization is influenced by the chemical structure of the sensitizing dye, by the properties of the emulsion (for example, composition of silver halides, crystal habit, crystal form, silver ion concentration) and other characteristics. It is also influenced by the photographic additives present in the emulsion, such as stabilizer agents, antifoggants, color couplers, etc.
Combination of the sensitizing dye and the coupler to form a salt, causing the sensitizer molecules to be desorbed from the grain surface, has been since long recognised in the art (H. Meier, Spectral Sensitization, The Focal Press, 1968, p.52).
2,5-Diacylamino-phenol cyan couplers are known to produce cyan dyes having excellent resistance against fading as described, for example, in U.S. Pat. Nos. 4,333,999; 4,451,559; 4,465,766 and 4,554,244.
The combination of 2,5-diacylamino-phenol cyan couplers with conventional cationic cyanine dyes shows no sensitometric problem if the coupler is added to a red sensitized emulsion immediately before coating, whereas it causes a dramatic loss of red sensitivity (desensitization) if the finalled emulsion is held several hours at high temperature before coating.
With cyanine dyes having two sulfoalkyl groups, desensitization does not occur, but such cyanine dyes cause an increased sensitization of the adjacent layers of the coated film (diffusion sensitization) since they can diffuse into them under high temperature and humidity. This is a serious problem in practical use.
U.S. Pat. No.4,513,081 describes a silver halide photographic emulsion comprising a 2,5-diacylamino-cyan coupler and a merocyanine sensitizing dye which does not undergo desensitization and diffusion sensitization. The inhibition of spectral sensitization which occurs with conventional carbocyanine dyes can be prevented by using particular merocyanine dyes.
U.S. Pat. No. 3,676,141 describes a process for dispersing an organic solution of a oleophilic photographic material into an aqueous gelatin solution in the presence of at least one anionic surface active agent and at least one nonionic surface active agent. The nonionic surface active agent used is selected from anhydrohexitol esters having HLB (Hydrophilic Lipophilic Balance) values in the range of from 3.5 to 9.5. The process provides finely dispersed oleophilic photographic materials and stable dispersions. 2,5-Diacylamino-phenol couplers are not disclosed in said patent.
U.S. Pat. No. 3,860,425 describes a process for dispersing an organic solution of an oleophilic photographic material in an aqueous medium in the presence of a) at least one nonionic surface active agent containing in the molecule thereof polyoxyethylene units and polyoxypropylene units in a molar ratio from 0.1 to 0.6 and b) at least one anionic surface active agent. Said noionic surface active agents are characterized by low values of HLB (for example, Pluronic.TM. L-61, a nonionic surface active agent made by Wyandotte Chem. Co. and cited at col. 11 lines 17-18, has an HLB of 3). The process provides stable dispersions in which the oleophilic photographic material is finely dispersed in the aqueous medium. 2,5-Diacylaminophenol couplers are not disclosed in said patent.
U.S. Pat. No. 3,912,517 describes a method for incorporating a water-insoluble photographic ingredient into a photographic hydrophilic colloid layer which comprises dissolving said ingredient in a water-soluble organic solvent, admixing said solution with water in the presence of a surface active agent but in the absence of a hydrophilic colloid, removing said organic solvent and incorporating the dispersion thus formed into the hydrophilic colloid coating composition. The surface active agent comprises a combination of an anionic dispersing agent and a non-ionic dispersing agent so that the HLB value of the combination is between 8 and 18 inclusive. Among the examples of nonionic surface active agents in said patent is Span.TM. 20 (trade name of the Atlas Chemical Industries Inc., USA, for sorbitan monolaurate). The process gives improved stability of the dispersion and better coating characteristics of the hydrophilic colloid layer including said dispersion. 2,5-Diacylamino-phenol couplers are not disclosed in said patent.